Arc welding current and voltage control method

ABSTRACT

A control method capable of effecting satisfactory arc welding by automatically controlling arc welding current and voltage. A processor calculates (106, 112) errors (εI, εV) between mean value (I, V) of actual welding currents and voltages periodically detected a predetermined number of times and target values IO, VO) of the welding current and voltage. If the welding current error falls outside an allowable range, a wire feeding speed correction amount is determined (ΔFW) (109) by substituting the mean value (I) of the welding currents and a welding current correction amount (ΔI) equivalent to the product of the calculated welding current error and a current gain into a calculation formula containing a first-degree polynomial (g&#39;(I)) for the welding current and a welding current change amount (ΔI) as variables. The wire feeding speed correction amount is input to a welding machine. If the welding voltage error falls outside an allowable range, the processor determines a power supply output correction amount (ΔU) (115) by substituting the welding current correction amount (ΔI) and a welding voltage correction amount (ΔV) equivalent to the product of the calculated welding voltage error and a voltage gain into a calculation formula containing welding current and voltage charge amounts (ΔI, ΔV) as variables. The power supply output correction amount is input to the welding machine. The welding machine adjusts the wire feeding speed and power supply output, to rationalize the welding current and voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to arc welding, and more particularly, toa control method capable of effecting satisfactory arc welding byautomatically controlling arc welding current and voltage.

2. Description of the Related Art

In order to adjust arc welding current and voltage for satisfactory arcwelding, trial welding is conventionally conducted by driving a weldingmachine with a tentatively set a power supply output and an electrodefeeding speed. If the welding current and voltage measured during thetrail welding are outside their respective allowable ranges, the trailwelding is executed again after resetting the power supply output andthe electrode feeding speed. Thus, according to the conventional method,the power supply output and the electrode feeding speed must be adjustedby the method of trial and error so that the welding current and voltagefall within their allowable ranges, while repeating the trail welding.Thus, the adjustment of the welding current and voltage requires labor.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a control methodcapable of effecting satisfactory arc welding by automaticallycontrolling arc welding current and voltage.

In order to achieve the above object, a welding current and voltagecontrol method according to the present invention comprises the stepsof: (a) detecting values of arc welding current and voltage; (b)calculating an error between the detected value of the arc weldingcurrent and a target value and an error between the detected value ofthe arc welding voltage and a target value; (c) calculating a correctionamount of an electrode feeding speed of an arc welding machine inaccordance with the calculated error of the arc welding current; and (d)calculating a correction amount of a power supply output of the arcwelding machine in accordance with the respective calculated errors ofthe arc welding current and voltage.

According to the present invention, as described above, the respectivecorrection amounts of the electrode feeding speed and the power supplyoutput of the arc welding machine are calculated in accordance with theerrors between the respective detected values and target values of thewelding current and the power supply output can be correctedautomatically to compensate the welding current and voltage errors,whereby the welding current and voltage can be automatically adjusted.Thus, there is no need of trail welding for the adjustment of thewelding current and voltage, and the arc welding can always be effectedsatisfactorily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating, by way of example, the weldingcurrent-welding voltage characteristic of an arc welding machine;

FIG. 2 is a graph illustrating, by way of example, the weldingcurrent-electrode feeding speed characteristic of the arc weldingmachine;

FIG. 3 is a schematic block diagram showing an arc welding robot forembodying an arc welding current and voltage feedback control methodaccording to one embodiment of the present invention; and

FIG. 4 is a flow chart showing a welding current and voltage feedbackcontrol process executed by means of the welding robot of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, a welding voltage V of a typical arc welding machinedrops straight and drastically as a welding current I increases in afirst welding current region where the welding current I takes a smallvalue. In a second (normal) welding current region where the weldingcurrent I takes a value greater than the value in the first weldingcurrent region, the welding voltage V drops straight in a rate of changelower than the rate of change in the first welding current region as thewelding current I increases. The greater the power supply output of thewelding machine, the higher the welding voltage. In FIG. 1, symbols U1and U2 (>U1) designate values of the power supply output. Thus, thewelding voltage V can be expressed as a function of the welding currentI and the power supply output U, as is given by equation (1).

    V=f(I, U).                                                 (1)

As shown in FIG. 2, the welding current I of the typical arc weldingmachine increases along a curve of second degree as an electrode feedingspeed FW increases. Thus, the electrode feeding speed FW can beexpressed as a function of the welding current I, as is given byequation (2).

    FW=g(I).                                                   (2)

The welding current-welding voltage characteristic and weldingcurrent-electrode feeding speed characteristic of the arc weldingmachine exemplarily illustrated in FIGS. 1 and 2 vary in dependence onwelding conditions including the type of the electrode (physicalproperties of the electrode, such as electrode material, electrodediameter, etc.).

The present invention has been originated in consideration of theaforesaid characteristics of the arc welding machine, and is intended tocorrect the electrode feeding speed and power supply output of the arcwelding machine, thereby rationalizing the arc welding current andvoltage. The following is a description of the principle of an arcwelding current and voltage control method according to the presentinvention.

First, totally differentiating equations (1) and (2), we obtain

    dV=(δf/δU)I·dU+(δf/δI)U·dI,(3)

    dFW=g'(I)dI.                                               (4)

Rearranging equations (3) and (4), we obtain

    ΔU={ΔV-(δf/δI)U·I}/(δf/δU)I,(5)

    ΔFW=g'(I)ΔI.                                   (6)

In the second welding current region of FIG. 1, the welding voltagelinearly changes with a change of the welding current, so that theparameter (δf/δI)U in equation (5), which is substantially constant, canbe represented by a constant K1. Further, two welding current-weldingvoltage curves corresponding individually to two supply outputs U1 andU2 shown in FIG. 1 extend parallel to each other in the second weldingcurrent region. Accordingly, the welding voltage linearly changes withpower supply output changes, so that the parameter (δf/δU)I in equation(5), which is substantially constant, can be represented by a constantK2. After all, equation (5) can be approximately replaced by equation(7).

    ΔU=(≢V-K1ΔI)/K2.                      (7)

Approximately, moreover, the welding current-electrode feeding speedcurve is formed of a curve of second degree, as shown in FIG. 2. Thus,the parameter g'(I) in equation (6) can be approximately replaced by afirst-degree curve (aI+b), and equation (6) by equation (8).

    ΔFW=g'(I)ΔI=(aI+b)ΔI.                    (8)

After all, the arc welding current and voltage change by ΔI and ΔV,respectively, if the power supply output and the electrode feeding speedchange by ΔU and ΔFW, respectively. Namely, the parameters ΔI and ΔV areregarded as required amounts of change of the welding current andvoltage, and the welding current and voltage can be adjusted to propervalues by changing the power supply output and the electrode feedingspeed by the change amount ΔU and ΔFW which fulfill equations (7) and(8), respectively.

The following is a description of the arc welding current and voltagefeedback control method according to one embodiment of the presentinvention.

Referring to FIG. 3, a welding robot for embodying the method accordingto the one embodiment of the present invention, which serves for arcwelding, including CO₂ welding, MIG welding, etc., comprises a controldevice 10 formed of, e.g., a numerical control device for controllingthe drive of a robot body 20 provided with an arm having the distal endthereof on which a welding torch (not shown) of the welding machine 30is fitted.

The numerical control device 10 comprises a processor (CPU) 11, aread-only memory (ROM) 12 loaded with a control program, and a randomaccess memory (RAM) 13 for storing a teaching program for robotoperation control and the result of arithmetic operation by means of theCPU 11. Further, the control device 10 comprises a teaching pendant 14used to create a teaching program, a control panel 15 used for manualoperation of the robot and data entry, an axis controller 16, aninterface 17, and servo circuits 18. The aforesaid elements 12 to 16 and17 are connected to the CPU 11 by means of busses 19. The servo circuits18 are connected in control relation to the axis controller 16 andservomotors (not shown) for individual axes of the robot body 20.Further, the interface 17 is connected to a power source, an electrode(wire) feeding device, a welding current detector, and a welding voltagedetector (none of which are shown) of the welding machine 30.

As mentioned before, the proper values (target values) of the weldingcurrent and voltage which enable satisfactory arc welding vary independence on the welding conditions including the type of the wire (notshown). Before the welding robot is operated in accordance with themethod of the present embodiment, therefore, the proper values of thewelding current and voltage corresponding to each set of the variouswelding conditions are experimentally determined in advance, forexample. At the same time, the power supply output (output current andoutput voltage of the power source) and the wire feeding speed whichallow the welding current and voltage of the proper values to beproduced are previously determined. Based on the result of theexperiment, moreover, those respective proper values of the parametersK1 and K2 associated with the power supply output and parameters a and bassociated with the wire feeding speed which correspond to the variouswelding conditions are determined in advance.

The following is a description of the operation of the welding robot ofFIG. 3.

An operator first determines the welding conditions including the typeof the wire used in the arc welding, then manually enters variousparameter values (target values of the welding current and voltage,tentative target values of the power supply output and the wire feedingspeed, set values K1 and K2 of the first and second power supply outputcorrection parameters, and set values a and b of the first and secondwire feeding speed correction parameters) for the welding current andvoltage control, which depend on the welding current and voltage intothe control device 10 through the control panel 15, and further startsthe welding robot.

In starting the robot, the CUP 11 of the control device 10 loads the RAM13 with the manually entered parameter values, and then starts a controloperation in accordance with the teaching program. More specifically,the CPU 11 starts position and attitude control for the welding torch bymeans of the axis controller 16, the servo circuits 18, and theservomotors for the individual axes, and delivers a welding command andthe tentative target values of the power supply output and the wirefeeding speed to the welding machine 30 via the interface 17. Further,the CPU 11 starts the welding current and voltage feedback controlprocess of FIG. 4. This control process is periodically executed bymeans of the CPU 11. At the start of the control process, the CPU 11resets each of stored values in first and second registers for storingcumulative detected values of the welding current and voltage and acounter value in a counter for counting the frequency of welding currentand voltage direction to the value "0."

In each processing period for the control process of FIG. 4, the CPU 11reads detected values I and V of an actual welding current and a voltagefrom the detectors of the welding machine 30 through the interface 17(Step 100), adds the detected values I and V to the stored values R(I)and R(V) in the first and second registers (Step 101), and incrementsthe counter value C in the counter by "1" (Step 102). Then, the CPU 11determines whether or not the counter value C is equal to apredetermined value C0 (Step 103). If the value C0 is not reached by thecounter value C, the processing for the present processing period ends.

If it is concluded in Step S103 in a subsequent processing period thatthe predetermined value C0 is reached by the counter value C, the CPU 11divides the cumulative detected values R(I) and R(V) of the actualwelding currents and voltages stored individually in the first andsecond registers by the predetermined value C0, thereby calculating themean values I and V of the actual welding currents and voltages (Steps104 and 105). Then, the CPU 11 subtracts the target value I0 of thewelding current from the mean value I of the actual welding currents,thereby calculating an error εI of the welding current (Step 106), anddetermines whether or not the absolute value |εI| of the error issmaller than a set value εI0, that is, whether or not the error εI fallswithin an allowable range (Step 107). If the welding current error εI iswithin the allowable range, each of the correction amount ΔI of thewelding current and the correction amount ΔFW of the target value of thewire feeding speed is set to the value "0" (Steps 110 and 111). If thewelding current error εI is deviated from the allowable range, on theother hand, the welding current correction amount ΔI is calculated bymultiplying the error εI by a preset current gain GI (Step 108). As isgiven by equation (8), moreover, the correction amount ΔFW of the targetvalue of the wire feeding speed is calculated by multiplying the sum ofthe set value b of the second wire feeding speed correction parameterand the product of the set value a of the first wire feeding speedcorrection parameter and the mean welding current value I by thecorrection amount ΔI (Step 109).

In Step 112 following Step 109 or 111, the CPU 11 calculates the errorεV of the welding voltage by subtracting the target value V0 of thewelding current from the mean value V of the actual welding voltages.Then, a determination is made as to whether or not the absolute value|εV| of the error is smaller than a set value εV0, i.e., as to whetheror not the error εV falls within an allowable range (Step 113). If thewelding voltage error εV is within the allowable range, the correctionamount ΔU of the target value of the power supply output is set to thevalue "0" Step 116). If the welding voltage error εV is deviated fromthe allowable range, on the other hand, the welding voltage correctionamount ΔV is calculated by multiplying the error εV by a preset voltagegain GV (Step 114). As is given by equation (7), moreover, thecorrection amount ΔU of the target value of the power supply output iscalculated by subtracting the product of the welding current correctionamount ΔI, calculated in its corresponding one of Steps 109 and 111, andthe set value K1 of the first power supply output correction parameterfrom the welding voltage correction amount ΔV calculated in Step 114,and by further dividing the subtraction result by the set value K2 ofthe second power supply output correction amount ΔV (Step 115).

Then, the CPU 11 calculates a new target value of the wire feeding speedby subtracting the correction amount ΔFW of the target value of the wirefeeding speed, calculated in its corresponding one of Steps 109 and 111,from the present target value FW of the wire feeding speed, and deliversthe updated target wire feeding speed value to the welding machine 30(Step 117). Likewise, the CPU 11 calculates a new target value U of thepower supply output by subtracting the correction amount ΔU of thetarget value of the power power supply output, calculated in itscorresponding one of Steps 115 and 116, from the present target value Uof the power supply output, and delivers the updated target power supplyoutput value to the welding machine 30 (Step 118). Further, each of thestored values R(I) and R(V) in the first and second registers and thecounter value C in the counter is reset to the value "0" (Step 119),whereupon the arc welding current and voltage control for the presentprocessing period ends.

During the arc welding, the control process of FIG. 4 is periodicallyexecuted, and the target values of the wire feeding speed and the powersupply output are updated or maintained so that the welding current andvoltage errors are compensated. The welding machine 30 adjusts the wirefeeding speed and the power supply output to the target values, so thatthe welding current and voltage are automatically controlled for thetarget values.

The present invention is not limited to the embodiment described above,and various modifications may be effected therein.

In the above embodiment, for example, the value of the variable g'(I) inthe wire feeding speed correction amount ΔFW(=b'(I)·ΔI) is calculated inaccordance with the calculation formula g'(I)=aI+b and using the meanvalue I the actual welding currents. Alternatively, however, values ofthe variable g'(I) may be tabulated beforehand in a manner correspondingindividually to a required number of welding current regions divided bywelding current values, and a variable value corresponding to the meanwelding current value I may be read out from the table.

We claim:
 1. An arc welding current and voltage control method for usein a robot control system, comprising the steps of:providing a weldingcurrent target value, a welding voltage target value, a power supplyoutput target value, a wire feeding speed target value, a weldingcurrent error range and a welding voltage error range; controlling apower source to output power in accordance with the power supply outputtarget value; controlling a wire feeding device to feed wire at a wirefeeding speed in accordance with the wire feeding speed target value;detecting an actual welding current and an actual welding voltage;calculating an actual welding current error, which is the differencebetween the actual welding current and the welding current target value;calculating an actual welding voltage error, which is the differencebetween the actual welding voltage and the welding voltage target value;calculating a power supply correction value based on the actual weldingvoltage error and the actual welding current error if the actual weldingvoltage error is outside of the welding voltage error range; calculatinga wire feeding speed correction value based on the actual weldingcurrent error if the actual welding current error is outside of thewelding current error range; supplying the power supply correction valueto the power source, which adjusts power supply output in accordancewith the power supply correction value; and supplying the wire feedingspeed correction value to the wire feeding device, which adjusts thewire feeding speed in accordance with the wire feeding speed correctionvalue.
 2. An arc welding current and voltage control method for use in arobot control system, comprising the steps of:inputting a weldingcurrent target value, a welding voltage target value, a power supplyoutput target value, a wire feeding speed target value, a weldingcurrent error range and a welding voltage error range into a controldevice having a memory and a CPU, which has a first register, a secondregister and a counter, wherein the welding current target value, thewelding voltage target value, the power supply output target value, thewire feeding speed target value, the welding current error range and thewelding voltage error range are stored in the memory of the controldevice; retrieving the power supply output target value from memory andtransmitting the power supply output target value to a power source,which outputs power in accordance with the power supply output targetvalue; retrieving the wire feeding speed target value from memory andtransmitting the wire feeding speed target value to a wire feedingdevice, which feeds wire at a wire feeding speed in accordance with thewire feeding speed target value; detecting an actual welding current,which is detected by a welding current detector; detecting an actualwelding voltage, which is detected by a voltage detector; reading theactual welding current from the current detector and adding the actualwelding current to value stored in the first register; reading theactual welding voltage from the voltage detector and adding the actualwelding voltage to value stored in the second register; incrementingvalue stored in the counter; repeating the detection and the addition ofactual welding current and actual welding voltage to the respectiveregisters as well as the incrementation of the counter until valuestored in the counter equals a predetermined counter value; determiningan actual welding current means value based on a summation of the actualwelding current detected by the current detector and stored in the firstregister if the value stored in the counter equals the predeterminedcounter value; determining an actual welding voltage mean value detectedby the welding voltage detector and stored in the second register if thevalue stored in the counter equals the predetermined counter value;retrieving the welding current target value and determining thedifference between the actual welding current mean value and the weldingcurrent target value to determine an actual welding current error;retrieving the welding voltage target value from memory and determiningthe difference between the actual welding voltage mean value and thewelding voltage target value to determine an actual welding voltageerror; retreiving the welding current error range from memory anddetermining whether the actual welding current error falls within thewelding current error range; retrieving the welding voltage error rangefrom memory and determining whether the actual welding voltage errorfalls within the welding voltage error range; calculating a wire feedingspeed correction value if the actual welding current error does not fallwithin the welding current error range; calculating a power supplycorrection value if the actual welding voltage error does not fallwithin the welding voltage error range; adjusting the power supplyoutput by the power supply correction value if a power supply correctionvalue is calculated; and adjusting the wire feeding speed of the wirefeeding device if a wire feeding speed correction value is calculated.